Math with previous row in SQL, avoiding nested queries? - sql

Question

I want to do some math on the previous rows in an SQL request in order to avoid doing it in my code.
I have a table representing the sales of two entities (the data represented here is doesn't make much sense and it's just an excerpt) :
YEAR ID SALES PURCHASE MARGIN
2009 1 10796820,57 2662369,19 8134451,38
2009 2 2472271,53 2066312,34 405959,19
2008 1 9641213,19 1223606,68 8417606,51
2008 2 3436363,86 2730035,19 706328,67
I want to know how the sales, purchase, margin... have evolved and compare one year to the previous one.
In short I want an SQL result with the evolutions pre-computed like this :
YEAR ID SALES SALES_EVOLUTION PURCHASE PURCHASE_EVOLUTION MARGIN MARGIN_EVOLUTION
2009 1 10796820,57 11,99 2662369,19 117,58 8134451,38 -3,36
2009 2 2472271,53 -28,06 2066312,34 -24,31 405959,19 -42,53
2008 1 9641213,19 1223606,68 8417606,51
2008 2 3436363,86 2730035,19 706328,67
I could do some ugly stuff :
SELECT *, YEAR, ID, SALES , (SALES/(SELECT SALES FROM TABLE WHERE YEAR = OUTER_TABLE.YEAR-1 AND ID = OUTER_TABLE.ID) -1)*100 as SALES_EVOLUTION (...)
FROM TABLE as OUTER_TABLE
ORDER BY YEAR DESC, ID ASC
But I have arround 20 fields for which I would have to do a nested query, meaning I would have a very huge and ugly query.
Is there a better way to do this, with less SQL ?

Answer 1

Using sql server (but this should work for almost any sql), with the table provided you can use a LEFT JOIN
DECLARE #Table TABLE(
[YEAR] INT,
ID INT,
SALES FLOAT,
PURCHASE FLOAT,
MARGIN FLOAT
)
INSERT INTO #Table ([YEAR],ID,SALES,PURCHASE,MARGIN) SELECT 2009,1,10796820.57,2662369.19,8134451.38
INSERT INTO #Table ([YEAR],ID,SALES,PURCHASE,MARGIN) SELECT 2009,2,2472271.53,2066312.34,405959.19
INSERT INTO #Table ([YEAR],ID,SALES,PURCHASE,MARGIN) SELECT 2008,1,9641213.19,1223606.68,8417606.51
INSERT INTO #Table ([YEAR],ID,SALES,PURCHASE,MARGIN) SELECT 2008,2,3436363.86,2730035.19,706328.67
SELECT cur.*,
((cur.Sales / prev.SALES) - 1) * 100
FROM #Table cur LEFT JOIN
#Table prev ON cur.ID = prev.ID AND cur.[YEAR] - 1 = prev.[YEAR]
The LEFT JOIN will allow you to still see values from 2008, where an INNER JOIN would not.

Answer 2

Old skool solution:
SELECT c.YEAR, c.ID, c.SALES, c.PURCHASE, c.MARGIN
, p.YEAR, p.ID, p.SALES, p.PURCHASE, p.MARGIN
FROM tab AS c -- current
INNER JOIN tab AS p -- previous
ON c.year = p.year - 1
AND c.id = p.id
If you have a db with analytical functions (MS SQL, Oracle) you can use the LEAD or LAG analytical functions, see http://www.oracle-base.com/articles/misc/LagLeadAnalyticFunctions.php
I think this would be the correct application:
SELECT c.YEAR, c.ID, c.SALES, c.PURCHASE, c.MARGIN
, LAG(c.YEAR, 1, 0) OVER (ORDER BY ID,YEAR)
, LAG(c.ID, 1, 0) OVER (ORDER BY ID,YEAR)
, LAG(c.SALES, 1, 0) OVER (ORDER BY ID,YEAR)
, LAG(c.PURCHASE, 1, 0) OVER (ORDER BY ID,YEAR)
, LAG(c.MARGIN, 1, 0) OVER (ORDER BY ID,YEAR)
FROM tab AS c -- current
(not really sure, haven't played with this enough)

Answer 3

You can do it like this:
SELECT t1.*, t1.YEAR, t1.ID, t1.SALES , ((t1.sales/t2.sales) -1) * 100 as SALES_EVOLUTION
(...)
FROM Table t1 JOIN Table t2 ON t1.Year = (t2.Year + 1) AND t1.Id = t2.Id
ORDER BY t1.YEAR DESC, t1.ID ASC
Now, if you want to compare more years, you'd have to do more joins, so it is a slightly ugly solution.